Copper strike plating bath

ABSTRACT

A copper strike plating bath comprising a copper cyanide compound and one or more of salts of inorganic acids and salts of organic acids. The copper cyanide compound is preferably sodium copper cyanide, potassium copper cyanide or a mixture thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a copper strike plating bath used for pretreating materials when plating copper-based or iron-based materials.

2. Description of the Related Art

Cyanide baths have conventionally been used to carry out copper strike plating on a plated material with satisfactory adhesion. For example, JP 11-274177 A describes the use of a copper strike plating bath comprising a mixture of sodium cyanide and copper cyanide when producing a lead frame by carrying out copper strike plating and partial silver plating.

In addition to being able to carry out plating with satisfactory adhesion on iron-based materials as well without dissolving the material, since the deposition potential of cyanide baths is sufficiently negative, they have superior characteristics along with surface activating effects accompanying hydrogen generation on a cathode surface. However, since plating current efficiency is greatly affected by changes in cyanide ion concentration in the bath, control of cyanide baths is difficult. In addition, if the metal salt concentration is increased to accommodate faster plating treatment, since the cyanide compound concentration of the conductive salt also increases simultaneously, the amounts of chemicals used increases. Since cyanide compounds are toxic, they are not desirable in terms of waste water treatment and the environment.

On the other hand, although there are copper pyrophosphate plating baths used for copper strike plating that do not use toxic cyanide compounds as described in JP 7-180085 A, these have a short bath life since the orthophosphoric acid ion concentration increases due to hydrolysis of pyrophosphate ions. In addition, since copper pyrophosphate plating baths involve deposition from divalent copper in contrast to cyanide baths involving deposition from monovalent copper, their deposition rate is slower.

Although copper sulfate baths are also used as strike plating baths that do not use cyanide as described in JP 7-180085 A, due to the occurrence of substitution between copper and iron in the case of plating iron-based materials, adhesion of the strike plating is inferior, and the deposition rate is slow similar to copper pyrophosphate baths.

In addition, strike plating baths are also known in which a chelating agent such as ethylene diamine is added to the bath. Although these baths do not have the problem of substitution, they are not desirable in terms of their burden on the environment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a copper strike plating bath that is capable of suppressing fluctuations in current efficiency and enables efficient copper strike plating while decreasing the burden on the environment.

The copper strike plating bath of the present invention comprises a copper cyanide compound and one or more of salts of inorganic acids and salts of organic acids.

The copper cyanide compound is preferably sodium copper cyanide, potassium copper cyanide or a mixture thereof.

The salt of an inorganic acid is preferably a phosphate, pyrophosphate, alkali hydroxide compound, borate or carbonate. The salt of an organic acid is preferably a salt of formic acid, acetic acid, tartaric acid, citric acid, gluconic acid or L-glutamic acid.

The copper strike plating bath of the present invention can additionally contain at least one inorganic acid or organic acid. The inorganic acid may be, for example, phosphoric acid, pyrophosphoric acid, boric acid or carbonic acid, while the organic acid may be, for example, formic acid, acetic acid, tartaric acid, citric acid, gluconic acid or L-glutamic acid.

The copper strike plating bath of the present invention may also additionally contain one or more of Se, Tl and Te, and can also contain a surfactant.

In the copper strike plating bath of the present invention, a conductive salt has a buffering action. Accordingly, there is decreased fluctuation in the pH during plating work, and the bath can be easily controlled. In addition, since the bath has high electrical conductivity, high anode-cathode polarizability and high current density, the use of the copper strike plating bath of the present invention makes it possible to improve macrothrowing power, or in other words, realize uniform plating deposition regardless of the shape of the plated material. In addition, since there are no large fluctuations in current efficiency caused by fluctuations in cyanide concentration as in cyanide baths of the prior art, the copper strike plating bath of the present invention can be easily controlled. Moreover, since the amount of cyanide compound used by the bath of the present invention can be made to be lower than that of baths of the prior art, waste water treatment costs can be decreased and the burden on the environment can be reduced.

In addition to these advantages, the copper strike plating bath of the present invention requires the metal salt concentration to be increased in order to increase the working current density. Therefore, it is easily compatible with cases of high-speed production of lead frames using, for example, continuous plating systems (in which various types of treatments, including plating, are carried out between unwinding from and winding back onto reels) that form a multilayer plated film on lead frames by continuously carrying out different types of plating in order.

DETAILED DESCRIPTION OF THE INVENTION

The copper strike plating bath of the present invention contains a copper cyanide compound as a copper supply source, and one or more of salts of inorganic acids and salts of organic acids as a conductive salt or salts.

Any compound that allows the deposition of copper on a plated material at a suitable rate can be used for the copper cyanide compound serving as the copper supply source. Typical examples of copper cyanide compounds include sodium copper cyanide and potassium copper cyanide. Two or more copper cyanide compounds can also be used. The copper strike plating bath of the present invention preferably contains 10 to 400 g/L, and more preferably 30 to 200 g/L, of copper cyanide compound.

The salt of an inorganic acid used as a conductive salt may be a phosphate, alkali hydroxide compound, borate, carbonate or the like. Examples of phosphates that can be used include ammonium salts such as ammonium hydrogen phosphate and triammonium phosphate, and potassium salts or sodium salts such as potassium dihydrogen phosphate or sodium dihydrogen phosphate, dipotassium hydrogen phosphate or disodium hydrogen phosphate and tripotassium phosphate or trisodium phosphate. Examples of pyrophosphates include potassium pyrophosphate and sodium pyrophosphate. Typical examples of alkali hydroxide compounds include sodium hydroxide and potassium hydroxide. Examples of borates that can be used include potassium borate, ammonium pentaborate, ammonium tetraborate, sodium borate, sodium tetraborate, potassium metaborate and sodium peroxoborate. Examples of carbonates that can be used include ammonium carbonate, ammonium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium carbonate and sodium hydrogen carbonate.

Examples of salts of organic acids used as a conductive salt include ammonium formate, potassium formate, sodium formate, ammonium acetate, potassium acetate, sodium acetate, ammonium tartrate, potassium tartrate, sodium tartrate, triammonium citrate, potassium dihydrogen citrate, tripotassium citrate, sodium dihydrogen citrate, trisodium citrate, ammonium gluconate, potassium gluconate, sodium gluconate, ammonium L-glutamate, potassium L-glutamate and sodium L-glutamate.

The copper strike plating bath of the present invention preferably contains 10 to 400 g/L, and more preferably 30 to 200 g/L, of inorganic acid salt or organic acid salt.

An acid can also be added to the copper strike plating bath of the present invention for the purpose of improving buffering action. Preferable acids are acids corresponding to the aforementioned salts, examples of which include phosphoric acid, pyrophosphoric acid, boric acid, carbonic acid, formic acid, acetic acid, tartaric acid, citric acid, gluconic acid and L-glutamic acid. Other acids in addition to these can also be used provided they do not have a detrimental effect on the performance of the copper strike plating bath. Two or more acids can also be used. Although the amount of acid added varies according to the characteristics of the copper strike plating bath to which the acid is added, it can typically be added at about 5 to 50 g/L.

The copper strike plating bath of the present invention can also contain other additives. For example, one or more of elements such as Se, Tl or Te may be added for the purpose of improving plating characteristics, or one or more surfactants (for example, polyethylene glycol) may be added.

EXAMPLES

The following provides a more detailed explanation of the present invention through its examples. It goes without saying that the present invention is not limited to the examples shown here.

Comparative Example 1

Potassium copper cyanide 185 g/L Potassium cyanide 10 to 80 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 8 A/dm² onto an iron-nickel alloy material in a copper strike plating bath having the above composition (the cyanide concentration changes according to the amount of potassium cyanide), it fluctuated from 10 to 90% depending on the cyanide concentration. In addition, the pH of the bath fluctuated from 9 to 12 depending on the cyanide concentration.

Comparative Example 2

Potassium copper cyanide 80 g/L Potassium cyanide 30 g/L

When current efficiency was measured during copper strike plating within a current density range of 3 to 12 A/dm² at 50° C. onto a copper alloy material in a copper strike plating bath having the above composition, current efficiency was 10% or less.

Comparative Example 3

Potassium copper cyanide 190 g/L Potassium cyanide  30 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 8 A/dm² onto a copper alloy material in a copper strike plating bath having the above composition, current efficiency was 50%.

Moreover, when copper plating was carried out at a thickness of 0.2 μm onto an iron-nickel alloy material using this bath, the resulting plating demonstrated a uniform, semi-bright appearance. When this test piece was heated on a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed. In addition, when the same heating test was carried out on a test piece subjected to copper strike plating at a thickness of 0.1 μm followed by silver plating at a thickness of 5 μm, there was no occurrence of discoloration, blistering or peeling of the silver plating.

Comparative Example 4

Copper pyrophosphate  16 g/L Potassium pyrophosphate 100 g/L Potassium oxalate  8 g/L

When copper strike plating was carried out under conditions of 50° C. and 8 A/dm² onto an iron-nickel alloy material in a copper strike plating bath having the above composition in accordance with JP 7-180085 A, the current efficiency was 25%, and black burned deposits were observed on plating having a thickness of 0.2 μm. When the current density was lowered to 2 A/dm², the burned deposits disappeared, and the current efficiency increased to 90% or more.

Moreover, when copper plating at a thickness of 0.2 μm was carried out at 2 A/dm² onto an iron-based alloy using this bath, although resulting plating had a satisfactory semi-bright finish, heating blisters formed when this test piece was subjected to a heating test with a hot plate at 400° C.

Example 1

Potassium copper cyanide 50 to 130 g/L Dipotassium hydrogen phosphate 100 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 8 A/dm² onto an iron-nickel alloy material in a copper strike plating bath having the above composition, it was from 20 to 50% depending on the cyanide concentration. The pH of the bath was from 9 to 10 depending on the cyanide concentration.

When copper plating was carried out at a thickness of 0.2 μm in these baths, all of the resulting plating demonstrated a satisfactory semi-bright appearance. Moreover, when these test pieces were subjected to heating tests with a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed.

Example 2

Potassium copper cyanide 80 g/L Potassium dihydrogen phosphate 50 g/L Dipotassium hydrogen phosphate 50 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 5 A/dm² onto a copper alloy material in a copper strike plating bath having the above composition, current efficiency was 45%. The pH of the bath was 7.

When copper plating was carried out at a thickness of 0.2 μm in this bath, the resulting plating demonstrated a satisfactory semi-bright appearance. Moreover, when this test piece was subjected to a heating test with a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed.

Example 3

Potassium copper cyanide 130 g/L Tripotassium citrate  20 g/L Dipotassium hydrogen phosphate  80 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 10 A/dm² onto a copper alloy material in a copper strike plating bath having the above composition, current efficiency was 40%. The pH of the bath was 8 to 9.

When copper plating was carried out at a thickness of 0.2 μm in this bath, the resulting plating demonstrated a satisfactory semi-bright appearance. Moreover, when this test piece was subjected to a heating test with a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed.

Example 4

Potassium copper cyanide  80 g/L Tripotassium citrate 100 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 5 A/dm² onto an iron-nickel alloy material in a copper strike plating bath having the above composition, current efficiency was 40%. The pH of the bath was 8 to 9.

When copper plating was carried out at a thickness of 0.2 μm in this bath, the resulting plating demonstrated a satisfactory semi-bright appearance. Moreover, when this test piece was subjected to a heating test with a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed. In addition, even when the tripotassium citrate was substituted with potassium tartrate, potassium acetate, potassium gluconate or potassium L-glutamate, current efficiency was nearly the same as that for tripotassium citrate, and adhesion was satisfactory.

Example 5

Potassium copper cyanide  80 g/L Dipotassium hydrogen phosphate 100 g/L

When current efficiency was measured during copper strike plating under conditions of 50° C. and 5 A/dm² onto a copper alloy material in a copper strike plating bath having the above composition, current efficiency was 40%. The pH of the bath was 9 to 10.

When copper plating was carried out at a thickness of 0.2 μm in this bath, the resulting plating demonstrated a satisfactory semi-bright appearance. Moreover, when this test piece was subjected to a heating test with a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed. When the same heating test was carried out on a test piece subjected to copper strike plating at a thickness of 0.1 μm followed by silver plating at a thickness of 5 μm, there was no occurrence of discoloration, blistering or peeling of the silver plating.

Example 6

Potassium copper cyanide 130 g/L Tripotassium citrate 20 g/L Dipotassium hydrogen phosphate 80 g/L Potassium selenocyanate 10 ppm

When current efficiency was measured during copper strike plating under conditions of 50° C. and 10 A/dm² onto an iron-nickel alloy material in a copper strike plating bath having the above composition, current efficiency was 50%. The pH of the bath was 8 to 9.

When copper plating was carried out at a thickness of 0.2 μm in this bath, the resulting plating demonstrated a satisfactory semi-bright appearance. Moreover, when this test piece was subjected to a heating test with a hot plate at 400° C., adhesion was satisfactory and heating blisters were not formed.

When Hull cell test was carried out under conditions of 5 A and 1 minute using Hull cell plates made of iron-nickel alloy and copper alloy for the copper strike plating baths of Comparative Example 3 and Examples 4 and 5, the resulting plating demonstrated a satisfactory uniform, non-bright to semi-bright appearance at all current densities. When these test pieces were subjected to heat testing with a hot plate at 450° C., heating blisters and peeling were not observed.

Hull cell test was carried out under conditions of 5 A and 1 minute on iron-based alloy Hull cell test pieces using a Hull cell for the copper strike baths of Comparative Example 3 and Examples 4 and 5, followed by an evaluation of macrothrowing power using the same method as the Watson method (Practical Plating for Engineers (I), Japan Plating Association, p. 433-434). More specifically, six points at distances of x=1.5, 2.5, 3.8, 4.8, 6 and 7.8 cm from the high current density side of the Hull cell test pieces were selected, and plating thicknesses t₁ to t₆ at the respective points were measured. In addition, the current densities of the respective points as shown in Table 1 were determined from the total current value. The combinations of primary current distribution ratios P₂, P₄ and P₅ shown in Table 1 were determined for the six measurement points, and the metal deposition ratios M₂, M₄ and M₅ corresponding to P₂, P₄ and P₅ were determined from the plating thicknesses t₁ to t₆ at the respective points. The macrothrowing power ratios T₂, T₄ and T₅ corresponding to P₂, P₄ and P₅ were then determined using the following equation. Those results are shown in Table 2. T=(P−M)/(P+M−2)×100

-   -   T: Macrothrowing power (%)     -   P: Primary current distribution ratio

M: Metal deposition ratio TABLE 1 Plating Current Primary current Metal thickness density distribution ratio deposition x (cm) (μm) (A/dm²) (P) ratio (M) 1.5 2.5 3.8 4.8 6 7.8 t₁t₂t₃t₄t₅t₆ 20 15 10 7.5 5 2

M₂ = t₂/t₄ M₄ = t₁/t₅ M₅ = #t₃/t₆

TABLE 2 Macrothrowing Comparative power Example 3 Example 4 Example 5 T₂ 77.4 76.3 79.1 T₄ 63.8 70.5 78.0 T₅ 61.6 71.2 73.3 (Units: %)

On the basis of these results, the macrothrowing powers of Examples 4 and 5 were demonstrated to be superior to the plating bath (conventional cyanide bath) of Comparative Example 3.

The copper strike plating bath of the present invention can be preferably applied to pretreatment when carrying out a plating treatment, such as silver plating, on products made of copper-based or iron-nickel alloy materials such as various products including lead frames mounted with semiconductor chips. 

1. A copper strike plating bath comprising a copper cyanide compound and one or more of salts of inorganic acids and salts of organic acids.
 2. The copper strike plating bath according to claim 1 wherein the copper cyanide compound is sodium copper cyanide, potassium copper cyanide or a mixture thereof.
 3. The copper strike plating bath according to claim 1 wherein the salt of an inorganic acid is a phosphate, pyrophosphate, alkali hydroxide compound, borate or carbonate.
 4. The copper strike plating bath according to claim 1 wherein the salt of an organic acid is a salt of formic acid, acetic acid, tartaric acid, citric acid, gluconic acid or L-glutamic acid.
 5. The copper strike plating bath according to claim 1 that additionally contains at least one inorganic acid or organic acid.
 6. The copper strike plating bath according to claim 5 wherein the inorganic acid is phosphoric acid, pyrophosphoric acid, boric acid or carbonic acid.
 7. The copper strike plating bath according to claim 5 wherein the organic acid is formic acid, acetic acid, tartaric acid, citric acid, gluconic acid or L-glutamic acid.
 8. The copper strike plating bath according to claim 1 that additionally contains one or more of Se, Tl and Te.
 9. The copper strike plating bath according to claim 1 that additionally contains a surfactant. 